1. Field of the Invention
The present invention broadly pertains to an image processing method and apparatus. More particularly, the present invention relates to an image processing method and apparatus, wherein image processing is carried out on color image signals, which have been obtained by reading a color image.
2. Description of the Prior Art
With techniques for reading a color image and reproducing the color image as a visible image, a color image having been recorded on photographic film, or the like, is photoelectrically read with a sensor, such as a charge coupled device (CCD), and red (R), green (G), and blue (B) three primary color image signals are thereby obtained. Various image processing operations are carried out on the image signals, and a visible image is reproduced on a recording material from the image signals, which have been obtained from the image processing. Systems for carrying out such techniques have been proposed, wherein a pre-scanning operation and a fine scanning operation are carried out. In the pre-scanning operation, a color image is scanned at coarse scanning intervals and is read photoelectrically. In this manner, the image information concerning the color image is approximately ascertained. Various parameters to be used in carrying out the image processing are set in accordance with the information, which has been obtained from the pre-scanning operation. Thereafter, the fine scanning operation is carried out. In the fine scanning operation, the color image is scanned at fine scanning intervals, and the R, G, and B three color image signals to be used for reproducing the visible image are obtained.
As the techniques for carrying out the image processing in the systems described above, for example, various techniques for enhancing the image sharpness by carrying out the image processing on the image signal, which represents a given image, have been proposed. As one of such techniques, a technique for enhancing the image sharpness by carrying out unsharp mask processing on the image signal has been proposed in, for example, xe2x80x9cImage Analysis Handbook,xe2x80x9d the publishing circle of the University of Tokyo, supervised by Mikio Takagi and Yokyu Shimoda, p. 549.
Also, a processing technique for suppressing the graininess of a reproduced image and enhancing the image sharpness has been proposed in, for example, U.S. Pat. No. 4,812,903. With the proposed processing technique, R, G, and B three color image signals are separated into a luminance signal and chrominance signals. Nonlinear processing is carried out on low frequency components of the luminance signal. Also, processing for enhancing high frequency components of the luminance signal is carried out. Thereafter, the luminance signal, which has been obtained from the processing, and the chrominance signals are combined together.
Further, a different image processing technique for enhancing the image sharpness and suppressing the image graininess has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 63-26783. With the proposed image processing technique, a luminance signal and chrominance signals (representing the hue, the saturation, or the like) are extracted from image signals, which represent a color image. Spatial filter processing is carried out to the luminance signal to thereby obtain spatial general information and spatial detail information. Predetermined enhancement processing is then carried on the spatial general information and the spatial detail information. Thereafter, the spatial general information and the spatial detail information, which have been obtained from the enhancement processing, are combined together, and a new luminance signal is thereby obtained. The new luminance signal and the chrominance signals are then combined together and converted into predetermined color image signals. With the proposed image processing technique, a processed image can be obtained, which has been subjected to natural sharpness enhancement processing such that little change in the color tone, or the like, may occur, and in which the graininess has been suppressed.
Furthermore, as for ordinary analog photographs, it has been known that, in cases where a photograph is enlarged from 35 mm film, or the like, to a size of 203 mmxc3x97254 mm, 254 mmxc3x97305 mm, or larger, patterns of grains in the film become perceptible, depending upon colors, and the image quality deteriorates. Therefore, an image processing technique for suppressing the graininess of the output image has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-277061. With the proposed image processing technique a predetermined color portion, such as a flesh-color portion, is extracted from a color image. In cases where the area of the predetermined color portion having thus been extracted becomes equal to a predetermined area or larger, noise removal processing is carried out on the predetermined color portion. In this manner, boundary lines between grains in the flesh-color or skin color region are removed, and the graininess of the output image is thereby suppressed.
Also, a technique for enhancing the image sharpness has been proposed in, for example, Japanese Patent Application Publication (Kokai) No. 3-502975. With the proposed technique, the image sharpness is enhanced by changing the value of a coefficient K in the formula for unsharp mask processing, which is shown below, in accordance with a characteristic portion of the image.
Sxe2x80x2=Sorg+Kxc2x7(Sorgxe2x88x92Sus)
wherein Sorg represents the original image signal, and Sus represents the unsharp mask signal. Specifically, with the proposed technique, local variance values, which are plotted with respect to the frequency of occurrence, are calculated as for a flat portion of the image, in which the level of noise due to the film graininess is high, a texture portion of the image, and an edge portion of the image. The value of the coefficient K is set as functions of the local area limited variance values.
However, with the techniques described above, a reproduced image having good image quality cannot be obtained. For example, with the unsharp mask processing described above, the sharpness can be enhanced. However, the unevenness due to the film graininess is also enhanced together with the enhancement of the sharpness. As a result, a reproduced image having good image quality and little noise cannot be obtained. With the technique proposed in U.S. Pat. No. 4,812,903 or Japanese Unexamined Patent Publication (Kokai) No. 63-26783, wherein the high frequency components of colors are not enhanced, the uneven feeling due to the film graininess can be suppressed to be lower than with the unsharp mask processing. However, unevenness of the luminance components due to the film graininess still remains unremoved. With the technique proposed in Japanese Unexamined Patent Publication No. (Kokai) 1-277061, noise due to the film graininess can be removed. However, the image sharpness cannot be enhanced and, as a result, a reproduced image which is easy to view cannot be obtained.
With the technique proposed in Japanese Patent Application Publication (Kokai) No. 3-502975, noise due to the film graininess can be suppressed, and the sharpness can be enhanced. However, the local variance values of the texture portion, the edge portion, or the like, in which the amplitude of the image signal is small, cannot be easily separated from the local variance values of the flat portion. Therefore, it often occurs that the texture and the edge, which are to be reproduced with a high sharpness, are suppressed in the same manner as that for noise in the flat portion.
In contrast, the applicant has proposed an image processing apparatus and method for suppressing graininess due to a film and enhancing sharpness in Japanese Unexamined Patent Publication (Kokai) No. 9-22460. In an image signal representing an ordinary image, the components, which have effects upon the sharpness of the reproduced image, are the high frequency components of the image signal. The film graininess, which appears as the unevenness in the reproduced image, is contained primarily in the middle frequency components. The present invention has been achieved on such findings. According to the invention, the image signal is separated into the low frequency components, the middle frequency components, and the high frequency components. The high frequency components are enhanced as well as the middle frequency components are suppressed to thereby enhance sharpness and suppress unevenness.
The above method is effective to suppress graininess due to an ordinary film and enhance sharpness. However, the method has a plenty of problems to be improved as to its effect on graininess which remains in the low frequency components, particularly in an under-exposed negative film, which is insufficiently exposed, and a high sensitive negative.
An object of the present invention which was made in view of the conventional problems is to provide an image processing method and apparatus capable of obtaining a reproduced image having good image quality by suppressing graininess and further improving the reproduced image particularly in the under-exposed negative film and the high sensitive negative.
To solve the above problems, the present invention provides an image processing method for carrying out graininess suppression to input image signals which have been obtained by photoelectrically reading a film original by an image reading device, comprising the steps of:
separating the input image signals into at least three kinds of frequency components composed of lowest frequency components a band of which is made more narrower, when it is determined that the film original is at least one of an under-exposed negative film and a high sensitive film, as compared with a case that the film original is neither the under-exposed negative film nor the high sensitive film, frequency components containing high frequency components and at least one kind of remaining frequency components;
enhancing the frequency components containing the high frequency components; as well as
suppressing at least one kind of frequency components of the at least one kind of the remaining frequency components; and
synthesizing the enhanced frequency components, the suppressed frequency components and the lowest frequency components.
It is preferable that the lowest frequency components are regarded as first low frequency components;
at least one kind of the remaining frequency components is separated into second low frequency components and middle frequency components in order of frequency; and
said input image signals are separated into the first low frequency components, the second low frequency components, the middle frequency components and the high frequency components in order of frequency.
The present invention also provides a method for performing image processing to carry out graininess suppression on input image signals which have been obtained by photoelectrically reading a film original by an image reading device, comprising the steps of:
when it is determined that said film original is at least one of an under-exposed negative film and a high sensitive film,
classifying the input image signals into four kinds of frequency components composed of first low frequency components having a lowest frequency band, second low frequency components, middle frequency components and high frequency components, as well as
separating the input image signals into at least three kinds of frequency components composed of the first low frequency components, frequency components containing at least the second low frequency components and frequency components containing at least the high frequency components;
performing enhancement processing on the frequency components containing at least the high frequency components;
performing suppression processing on the frequency components containing at least the second low frequency components; and
synthesizing both kinds of the processed frequency components and the first low frequency components.
It is preferable to create the first low frequency components using an IIR filter.
It is preferable to change the lowest band of the first low frequency components in accordance with a print size of a print which is printed out after the image processing is carried out.
It is preferable that the frequency components are distributed in a pattern such that they may have a peek at a Nyquist frequency of output in reproduction of a visible image from image signals having been processed,
the middle frequency components are distributed in a pattern such that they may have a peek in a vicinity of ⅓ to the Nyquist frequency of the output,
the first low frequency components are lower halves of low frequency components that are distributed in a pattern such that they may have a peek at a frequency of zero, and
the second low frequency components are upper halves of said low frequency components.
It is preferable that the high frequency components contain information concerning edge and fine texture portions of a color image of the film original,
the middle frequency components contain unevenness due to graininess of the film original,
the first low frequency components contain neither the information concerning the edge and fine texture portions of the color image of the film original nor the unevenness due to the graininess of the film original, and
the second frequency components contain no information concerning the edge and fine texture portions of the color image of the film original and the unevenness due to the graininess of the film original in case that the film original is at lest one of the under-exposed negative film and the high sensitive film.
It is preferable that luminance components are extracted from the high and middle frequency components and the enhancement processing and suppression processing are performed based on the luminance components.
It is preferable that the image processing for carrying out the graininess suppression can be switched to one of soft focus processing and dodging processing by modes which are previously set.
It is preferable that, when it is determined that the film original is neither the under-exposed negative film nor the high sensitive film, the input image signals are separated into three kinds of the frequency components composed of a low frequency components containing the first and second frequency components, the middle frequency components, the high frequency components, the enhancement processing is performed on the high frequency components, the suppression processing is performed on the middle frequency components and both processed frequency components and the low frequency components are synthesized.
To solve the problem described above, the present invention provides an image processing apparatus for carrying out graininess suppression to input image signals which has been obtained by photoelectrically reading a film original by an image reading device, comprising:
determination means for determining that the film original is at least one of an under-exposed negative film and a high sensitive film;
separation means for separating the input image signals into at least three kinds of frequency components composed of lowest frequency components a band of which is made more narrower, when it is determined by the determination means that the film original is at least one of the under-exposed negative film and the high sensitive film, as compared with a case that the film original is neither the under-exposed negative film nor the high sensitive film, frequency components containing high frequency components and at least one kind of remaining frequency components;
enhancement means for enhancing the frequency components containing the high frequency component;
suppression means for suppressing at least one kind of frequency components of the at least one kind of the remaining frequency components; and
synthesis means for synthesizing the enhanced frequency components, the suppressed frequency components and the lowest frequency components.
It is preferable that the separation means regards the lowest frequency components as first low frequency components, separates the at least one kind of the remaining frequency components into second low frequency components and middle frequency components in order of frequency and separates the input image signals into the first low frequency components, the second frequency components, the middle frequency components and the high frequency components in order of frequency.
The present invention also provides an apparatus for performing image processing to carry out graininess suppression on input image signals which have been obtained by photoelectrically reading a film original by an image reading device, comprising:
determination means for determining that the film original is at least one of an under-exposed negative film and a high sensitive film;
classification means for classifying the input image signals into four kinds of frequency components composed of first low frequency components having a lowest frequency band, second low frequency components, middle frequency components and high frequency components;
separation means for separating the input image signals into at least three kinds of frequency components composed of the first low frequency components, frequency components containing at least the second low frequency components and frequency components containing at least the high frequency components, when determined that the film original is at least one of the under-exposed negative film and the high sensitive film;
enhancement processing means for performing enhancement processing on the frequency components containing at least the high frequency components;
suppression processing means for performing suppression processing on the frequency components containing at least second low frequency components; and
synthesis means for synthesizing both kinds of the processed frequency components and the first low frequency components.
It is preferable that the separation means creates the first low frequency components using an IIR filter.
It is preferable that the apparatus further comprises changing means for changing the lowest band of the first lower frequency components in accordance with a print size of a print which is printed out after the image processing is carried out.
It is preferable that the frequency components are distributed in a pattern such that they may have a peek at a Nyquist frequency of output in reproduction of a visible image from image signals having been processed,
the middle frequency components are distributed in a pattern such that they may have a peek in a vicinity of ⅓ to the Nyquist frequency of the output,
the first low frequency components are lower halves of low frequency components that are distributed in a pattern such that they may have a peek at a frequency of zero, and
the second low frequency components are upper halves of said low frequency components.
It is preferable that the high frequency components contain information concerning edge and fine texture portions of a color image of the film original,
the middle frequency components contain unevenness due to graininess of the film original,
the first low frequency components contain neither the information concerning the edge and fine texture portions of the color image of the film original nor the unevenness due to the graininess of the film original, and
the second frequency components contain no information concerning the edge and fine texture portions of the color image of the film original and the unevenness due to the graininess of the film original in case that the film original is at least one of the under-exposed negative film and the high sensitive film.
It is preferable that the apparatus further comprises extracting means for extracting luminance components from the high and middle frequency components, wherein the enhancement processing means and the suppression means perform the enhancement processing and suppression processing based on the luminance components extracted by the extracting means, respectively.
Further, it is preferable that the image processing apparatus comprises switching means for switching modes from the image processing for carrying out the graininess suppression to one of soft focus processing and dodging processing which are previously set as the modes.
It is preferable that, when it is determined by the determination means that the film original is neither the under-exposed negative film nor the high sensitive film, the separation means separates the input image signals into three kinds of the frequency components composed of a low frequency components containing the first and second frequency components, the middle frequency components, the high frequency components, the enhancement processing means performs the enhancement processing on the high frequency components, the suppression processing means performs the suppression processing on the middle frequency components and the synthesis means synthesizes both processed frequency components and the low frequency components.